Surveillance method and system with electromagnetic carrier and

Details Surveillance method and system with electromagnetic carrier and Surveillance method and system with electromagnetic carrier and

1977-10-07

1979-02-13

Swann, III, Glen R.

Communications: electrical

Condition responsive indicating system

Specific condition

343 68R, G08B 1318

Patent

active

041398440

ABSTRACT:
A microwave carrier signal is transmitted throughout a surveillance field. Two low frequency signals are also radiated into the field from opposite sides to limit the field size. A miniature receptor-reradiator containing a nonlinear signal mixing device responds to the signals when in the field to return a modulated carrier signal to a receiver device for actuating an alarm. The alarm is actuated only when the receiver detects a signal having a carrier component corresponding to the microwave signal with sideband components corresponding to the sum of the frequencies of the two low frequency signals.

REFERENCES:
patent: 3707711 (1972-12-01), Cole et al.
patent: 3895368 (1975-07-01), Gordon et al.

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